Bacillus is a genus of rod-shaped bacteria. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species. There are three important pathogenic members of Bacillus: B. cereus causes a foodborne illness similar to that of Staphylococcus; B. thuringiensis is an important insect pathogen, and is sometimes used to control insect pests; and B. anthracis causes anthrax in humans and animals. In humans, three types of anthrax occur: cutaneous, gastrointestinal, and pulmonary anthrax. Under stressful environmental conditions, these cells shift to an alternative developmental pathway, sporulation, and produce oval endospores that can stay dormant for extended periods. Bacillus anthracis is infectious in the endospore form.
Much of what is known of the sporulation process comes from genetic studies of the nonpathogenic Bacillus subtilis. Unlike the pathogenic Bacillus spp. above, B. subtilis does not have an exosporium layer on its exterior. This outermost layer of the endospore consists of a basal layer surrounded by an external nap of hair-like projections (see FIG. 1). Filaments of the hair-like nap are predominantly formed by the collagen-like glycoprotein BclA, while the basal layer is comprised of a number of different proteins.
Relatively little is known about the exosporium layer because of the difficulty in genetically manipulating the exosporium-containing species. Several proteins have been identified as being associated with the exosporium, or their loss is associated with defects in exosporium assembly. The components of the outer spore layers are thought to be produced in the mother cell and then assembled on the developing spore. Many of these components are expressed during the sporulation cycle by RNA polymerase bearing the σκ sigma factor. Although the exosporium layer does not appear to be essential for virulence and does not confer the principal resistance properties to the spore, its location on the spore suggests that it plays a role in initial host-spore interactions. It is also an important source of spore antigens which may enhance the protective effect of current vaccines.
While spores of B. subtilis have been proposed as vaccine delivery vehicles using CotB and CotC fusion proteins, such a system is unlikely to function with the B. cereus family of bacteria, i.e. B. anthracis, B. cereus, and B. thuringiensis. These bacteria include the exosporium layer that is likely to prevent fusions to CotB or CotC from being exposed on the surface and available as delivery vehicles. Because anthrax, especially pulmonary anthrax, is an acute, fatal disease and may potentially be utilized as a bioterrorism weapon, a system to delivery immunogenic antigens or other therapeutic molecules effective against anthrax is desired. Herein, such a system is described.